Information systems for storage devices, general-purpose computers and servers use a large number of DRAMs (Dynamic Random Access Memory) which are volatile memories. Therefore, when power supply from the exterior is interrupted, the data stored in the volatile memory is lost. Therefore, the information system saves the contents of stored data during interruption of power supply either by feeding power to a volatile memory using a data backup battery and saving data in the volatile memory or by storing data in a nonvolatile memory for saving data.
Incidentally, there are a short-time blackout and a long-time blackout in the interruption of power supply. The short-time blackout includes “instantaneous voltage drop”, “momentarily blackout” and “instantaneous blackout”. “Instantaneous voltage drop” refers to a state where the voltage of a line power supply drops instantaneously (for several milliseconds to several seconds). “Momentarily blackout” refers to a state where the line power supply is turned off instantaneously (for several milliseconds to several seconds). “Instantaneous blackout” refers to a state where the line power supply is turned off instantaneously (for several milliseconds to several seconds), which is generally longer than momentarily blackout.
In addition, the short-time blackout generally occurs more frequently than the long-time blackout. According to the state of occurrence of instantaneous voltage drops in Japan (the art disclosed in patent literature 2), an instantaneous voltage drop with a 20% voltage drop ratio and approximately three-cycle continuous time most frequently occurs. The blackout characteristics, such as the blackout type or the total time of occurrence of blackout per year, differ among countries and regions.
Patent literatures 1 through 3 disclose examples of the prior art mentioned above.
The art disclosed in patent literature 1 relates to an information processing system which saves data in a volatile memory to a nonvolatile memory when power is off. The information processing system comprises a detecting circuit for outputting a momentary interruption detecting signal when a power source voltage is below a first threshold voltage, and a power failure detecting signal when the power source voltage is below a second threshold voltage that is lower than the first threshold voltage. When the detecting circuit has output the momentary interruption detecting signal, the information processing system starts saving the data in the volatile memory into the nonvolatile memory. Thereafter, when the detecting circuit has output the power failure detecting signal, the data processing apparatus continues the saving of the data while carrying out a predetermined shutdown processing.
According to the disk array device (backup system) disclosed in patent literature 2, when a blackout occurs, backup of data is performed via a first backup method (performing self-refreshing of the volatile memory via the power supplied from a battery to store the data in the memory). The disk array device integrates the backup time, the battery discharge currents and the like via the first backup method. Thereafter, at a timing when the integrated value satisfies a predetermined condition, the disk array device switches the backup method from the first backup method to a second backup method and evacuates data from the volatile memory to a nonvolatile memory.
Patent literature 3 discloses a semiconductor disk device having a first volatile memory, a second volatile memory, and a third non-volatile memory having the same capacity, so as to maintain the completeness and consistency (uniqueness) of data evacuation. When power is conducted from an external power supply, the semiconductor disk device forms a duplex structure via the first volatile memory and the second volatile memory. When blackout occurs, the semiconductor disk device releases the duplex structure and saves the contents of the second volatile memory to the third nonvolatile memory. When the external power supply is restored during the saving operation, the first volatile memory performs data transfer to an upper level host in response to a reading or writing command from the upper level host.